Utility type: Object entries tuple

Search Terms

Utility type, object entries, tuple, mapped type

Suggestion

An Entries<T> utility type that constructs a union of all object entry tuples of type T in lib.es5.d.ts.

/**
 * Construct a union of all object entry tuples of type T
 */
type Entries<T> = {
    [P in keyof T]: [P, T[P]];
}[keyof T];

Use Cases

I understand the history behind why Object.entries provides string as the key type rather than the actual keys (because there may exist more actual keys that those defined in the type). I refer to discussion here.

Sometimes, however, we can be confident that only properties defined in the type will be present at runtime, and have found that a utility type like this can be helpful to more strictly define such types, particularly in tests.

Examples

const expected = {
   foo: "foo",
   bar: "Bar",
   baz: "BAZ",
} as const;

(Object.entries(expected) as Entries<typeof expected>).forEach(([input, output]) => {
  // where `myFunction`'s first parameter only accepts `"foo" | "bar" | "baz"` (or a superset)
  expect(myFunction(input)).to.equal(output);
});

function thatAcceptsJsxAttributeTuples(attributes: readonly Entries<JSX.IntrinsicElements["path"][]>): void {
  // ...
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

La proposition cible lib.es5.d.ts ; commencez par là et comparez les déclarations existantes de utility types avec la forme suggérée Entries. C’est terminé lorsque le utility type est ajouté de manière cohérente avec la bibliothèque standard et prend en charge les exemples de tuples d’entrées d’objet décrits dans l’issue.